1. Field of the Invention
The present invention relates to a method for dicing a wafer, particularly to a method for dicing and separating a thin wafer in the process of manufacturing, for example, a non-contact type IC card.
2. Description of the Related Art
Silicon wafers of the thin film type have been used in a variety of fields, such as in the field of manufacturing a non-contact type IC card. Non-contact type IC cards have been used in a lot of fields, such as the application to ticket gates of railroads, and are expected to be applied in a lot of fields in the future. A non-contact type IC card has an antenna, a semiconductor chip (IC chip), and a sheet material to cover the antenna and the IC chip. Silicon wafers of the thin film type are also used for the manufacturing of cellular phones and various types of sensors.
The reasons for using thin silicon wafers in these products are as follows. The thickness of a wafer is related to the thickness of IC cards which are to be produced when a wafer is used for IC cards. Moreover, thin silicon wafers are advantageous because of their flexibility which allows them to be bent without causing any damage during use. ICs, when applied to cellular phones, need compact and three-dimensional mounting within a small space by methods such as piggyback stacking. For these reasons, IC chips are required to be processed as thin as possible. ICs, when applied to, for example, sensors, are required to be processed thin to ensure mounting of the chips, as well as to ensure a reduction in chip size.
An IC chip consists of a chip portion, which has a base plate made of stacked films of silicon etc. to form circuits thereon, and a reinforcing plate, which is normally made of stainless steel, to support the chip portion. The reinforcing plate has the function of protecting the chip portion against excessive deformation and of a heat sink which absorbs heat that is generated in the chip portion.
Various methods for manufacturing an IC chip have been proposed. According to a known art, deep grooves are formed in a wafer prior to dicing the wafer. In this method, first, deep grooves are formed along the chip to a depth that is larger than the thickness of the finished wafer. Next, the wafer is attached to a base plate via an adhesive tape or wax that is applied on the base. The wafer is then ground to a predetermined thickness from the back side of the wafer to separate the wafer into chips. However, there are some drawbacks in this method because of the internal stress and internal distortion which are induced within the wafer due to the machining on the side which is ground. First, the chips are more apt to have cracks due to external forces. Secondly, as the chips become thinner, their characteristics, as an element, tend to be more affected. In particular, the characteristics may be significantly deteriorated when the chip is used for, for example, as a magnetic sensor. Furthermore, as chips become smaller, chips tend to peel from the base plate during grinding, or tend to experience chipping because of the vibration and displacement during grinding. The term ‘chipping’ means a crack in a wafer along a dicing line.
Therefore, another process may be used in which a wafer is diced into chips after the wafer is made thinner. In this method, a wafer having a plurality of chip portions thereon is made thinner from the back side of a wafer, and then the thin wafer is diced into individual chip portions. Finally, the separated chip portions are attached to reinforcing plates in order to manufacture IC chips.
Specifically, this method is carried out in accordance with the following steps. First, a wafer is attached to a base plate, and then ground without being separated into chips at this step.
The surface may be etched after grinding in order to remove the distortion that is generated on the surface through the machining process. Etching may be performed through wet etching using fluoro-acid etc., or through milling in a vacuum, or through reactive etching in a vacuum such as RIE. However, in this method, it is not desirable to form grooves first for the following reasons. In a wet process, as well as in the reactive etching process, the etching solution may flow out to the opposite side of the wafer, causing damage on the surface of the elements. In a physical etching process such as milling, the elements are more apt to be damaged by the heat that is generated during milling. Removing the stress by means of etching etc. using such a process after the wafer is made thinner facilitates obtaining flat chips after the chips are removed from the base plate. In this way, chips are uniformly attached onto base plates, such as heat sinks, and become more resistant to cracks which are generated by external forces. When the elements are used, for example, as sensors, their characteristics, as an element, are less affected.
Next, a plastic sheet made of polyester or the like is placed on a dicing base. The plastic sheet is then fixed on the dicing base with the help of negative pressure on the back side of the plastic sheet. Next, resin, such as ultraviolet curing resin or thermosetting resin, is applied on the plastic sheet, as shown in the specifications etc. of Japanese Patent Laid-open Publication No. 2002-270676 and 92778/98. Such a stacked structure formed in this manner consisting of the plastic sheet and the resin is called a processing tape.
The reason for applying resin on a wafer is as follows. A wafer, which is diced along the boundaries between the chip portions, has an exposed portion of the base plate made of silicon etc. along the boundaries, and terminals for inspection on the back side of the chip portions, and the brittleness of the terminals and the base plate facilitates the occurrence of chipping. Although the chip portion is covered with a protective film made of materials such as polyimide, foreign substances, such as gas or water, will invade the chip portion to cause corrosion, when chipping at the boundary reaches the protective film. Moreover, the concentration of chipping may lead to stress concentration, which causes cracks which grow from the location of the chipping during temperature cycles and which result, in the worst case, in the fracture of the element. For this reason, it is important to prevent the chipping while dicing a wafer. The application of resin, such as ultraviolet curing resin or thermosetting resin, reduces resistance against dicing, which may cause chipping, and also stabilizes (prevents the deflection of) the wafer and the grindstone during dicing. In addition, removal of resin, which becomes unnecessary after dicing, can be facilitated.
Next, the processing tape is held by a fixing device, called a dicer ring, and stretch (tension) is applied to the processing tape in order to prevent air from entering the space between the dicing base and the processing tape, and thereby to make the processing tape flat. Next, a wafer is attached to the processing tape on the side of the resin. These processes can be automatically performed by special equipment called a mounter.
Next, the wafer having the processing sheet thereon is diced by a grindstone which has abrasive grains made of, for example, artificial diamond. The chipping depends largely on the particle size (The particle size is the index of the particle diameter of grains in a grindstone. A grain size number means the density of grains per one square inch. The grain size number is in proportion to the number of the grains, and is in inverse proportion to the diameter of the particle.) A grindstone with large particle diameters tends to get stuck in a wafer and to cause chipping because of the large resistance against dicing. Thus, in general, a grindstone having particles with small diameters, or particles with large particle number, is used to dice a thin wafer, which is apt to have chipping. In the field of semiconductors, grindstones with a grain size number between 2000s and 4000s are used.
The processing sheet is peeled after dicing the wafer. In order to peel the processing sheet, according to a disclosed method, a needle is stuck to the back surface of the processing sheet in order to partially separate the chip portion from the processing sheet, and then the chip portion is picked up by vacuum tweezers etc. See, for example, the specification etc. of Japanese Patent Laid-open Publication No. 151265/94.
Thus, the method of dicing a wafer into chips after making the wafer thinner, which enables improvement in the reliability and uniformity in the characteristics of the separated chips, has been used more frequently in line with the trend toward reduced chip size and increased chip density.
Recently, a dicing technology which uses laser also has been widely studied.
However, there are the following drawbacks in prior art. Specifically, wafers have become increasingly thinner in recent years as mentioned above. However, as a wafer becomes thinner, a wafer is apt to move up and down during dicing, and the grindstone experiences unstable movement, resulting in the increased possibility of fine cracks, i.e., chipping. The result is the same even if a protection tape mentioned above is used. Furthermore, a wafer the surface of which is etched after being made thinner, becomes significantly less resistant against peeling during dicing, because the capacity to hold the wafer during dicing, which is due to the thickness of the base plate, is weakened. If a wafer is attached to a base plate with a tougher adhesive in order to improve resistance against peeling, removal of chips becomes more difficult, resulting in a significant reduction in yield in the separating process after dicing, as well as resulting in damage to the chips.
Grindstones with larger grain size numbers may be used in order to reduce the resistance against dicing. However, the method is economically disadvantageous, because grindstones become more expensive as the grain size number is increased. Alternatively, resistance against dicing can be suppressed by reducing dicing speed, i.e., by reducing dicing speed to an extremely low speed at first (for example, about 5 mm/sec), then by gradually increasing the dicing speed while checking the dicing condition to confirm whether there are problems or not. However, this method worsens working efficiency because of the need to check the dicing condition and because of the low average dicing speed.
In addition, the method for dicing a wafer that uses a laser is disadvantageous in that the heat generated during irradiation may damage a wafer, especially a thin wafer.
Thus, there remain major problems of how to prevent chipping when a thin wafer is diced. It is pointed out that the problem of chipping becomes significant especially for a wafer that is less than 0.1 mm in thickness, as mentioned, for example, in the specification etc. of Japanese Patent Laid-open Publication No. 2002-270676.
Moreover, because tension in the processing tape and in the resin film as well, that is given after the application of resin, is released such that the resin film deforms to shrink locally as soon as the resin film is diced, the dicing plane of a wafer moves during dicing, causing, in the worst case, cracks on the wafer or on elements formed on the wafer.